1. Field of the Invention
The present invention relates to a nonaqueous electrolyte battery used in, e.g., a power supply of an electronic apparatus and, more particularly, to a nonaqueous electrolyte battery having an anode, a cathode, and an electrolyte solution, all of which are sealed in a film case, an anode lead wire, and a cathode lead wire.
2. Related Background Art
Recently, rapid miniaturization of electronic apparatuses, particularly portable electronic apparatuses, is remarkably advancing. Accordingly, the development and practical use of high-performance power supplies which are small in size and light in weight and have high energy density are in strong demand.
In particular, a nonaqueous electrolyte battery, such as a lithium ion secondary battery, in which an anode, a cathode, and an electrolyte solution are sealed in a small and light film case is expected as a power supply of the small electronic apparatuses described above, since the battery has high battery voltage and high energy density and can be easily made compact and light in weight.
For this nonaqueous electrolyte battery, one important point which makes long battery life and long-term use possible is to increase the sealing properties of a nonaqueous electrolyte and thereby prevent decomposition of the nonaqueous electrolyte when it comes into contact with moisture.
As an attempt to improve the sealing properties of a nonaqueous electrolyte, Japanese Patent Laid-Open Nos. 61-240564, 3-62447, and 57-115820, for example, have disclosed the use of a film case manufactured by heat-sealing a packaging film including an acid-resistant thermoplastic polymer layer and a metallic layer. However, it is difficult to completely prevent an invasion of moisture from the heat-sealed portion of the film case only by the use of the acid-resistant thermoplastic polymer layer and the metallic layer. Moisture entering the battery from the outside reacts with the electrolyte solution to produce an acid, and this acid penetrates through the thermoplastic polymer layer to corrode the metallic layer. This corrosion of the metallic layer causes peeling between this metallic layer and the thermoplastic polymer layer, leading to a leak of the solution.
To prevent this, Japanese Patent Laid-Open No. 10-208709 has proposed a nonaqueous electrolyte battery which uses a packaging film in which a thermoplastic polymer layer containing metal compound particles is formed inside a metallic layer.
Unfortunately, even this nonaqueous electrolyte battery described in Japanese Patent Laid-Open No. 10-208709 is still unsatisfactory because in some cases it cannot be used for long time periods.
The present inventors studied why this nonaqueous electrolyte battery described in Japanese Patent Laid-Open No. 10-208709 cannot be well used for long time, and have found the reason.
That is, the intention of this prior application is to allow the metal compound contained in the thermoplastic polymer layer to absorb and decompose acid produced in a nonaqueous electrolyte solution before the acid moves to the metallic layer. This metal compound is conventionally contained in an amount of about 30 wt % in order to prevent corrosion of the metallic layer and prevent peeling between the metallic layer and the thermoplastic polymer layer caused by the corrosion of the metallic layer. However, when the content of the metal compound in the thermoplastic polymer layer is thus high, the strength of the thermoplastic polymer layer lowers to permit easy progress of deterioration such as cracking. Additionally, the adhesion of the heat-sealed portion of the packaging film becomes insufficient to promote an invasion of moisture from the outside.
Also, when cracking or the like occurs as described above, the acid produced in the nonaqueous electrolyte solution readily moves to the metallic layer. This accelerates the corrosion of the metallic layer and the peeling between this metallic layer and the thermoplastic polymer layer resulting from the corrosion of the metallic layer.
Furthermore, the metal compound particles not only absorb and decompose the acid moving from the nonaqueous electrolyte solution but also adsorb water produced by the reaction between this acid and the metal compound particles, thereby preventing the water from leaking out of the thermoplastic polymer layer. However, when the content of the metal compound in the thermoplastic polymer layer is high as described above and hence deterioration such as cracking progresses in this thermoplastic polymer layer, this moisture adsorbing function of the metal compound particles does not effectively function. Consequently, the produced water leaks out of the thermoplastic polymer layer to promote production of more acid in the nonaqueous electrolyte solution or comes in contact with the metallic layer, as acidic water containing an acid, thereby accelerate the corrosion of the metallic layer and the peeling between the metallic layer and the thermoplastic polymer layer.
Especially in a lithium ion secondary battery containing a fluorine-based lithium salt such as LiPF6 in an electrolyte solution, highly corrosive hydrofluoric acid is produced by the reaction between water and this nonaqueous electrolyte solution. As a consequence, the above problem appears more significantly.
The present invention has been made in consideration of the above situation, and has as its object to provide a nonaqueous electrolyte battery having high sealing properties of an electrolyte solution and capable of being used for long time periods, even if acid is produced by an invasion of moisture, by sufficiently and reliably preventing corrosion of a metallic layer and detachment between this metallic layer and a thermoplastic polymer layer caused by the corrosion of the metallic layer over long periods of time.
The present inventors made extensive studies to achieve the above object and have reached the present invention by finding that when about 2 parts by weight to about 25 parts by weight (%) of a metal compound are contained in 100 parts by weight of a thermoplastic polymer and a water-proof thermoplastic polymer layer is formed inside the thermoplastic polymer layer (acid resistant layer) containing this metal compound, it is possible to sufficiently and reliably maintain the absorptivity to acid and water of the thermoplastic polymer layer containing the metal compound, the strength of the thermoplastic polymer layer, and the adhesion of a heat-sealed portion of the thermoplastic polymer layer over long time periods.
That is, a nonaqueous electrolyte battery of the present invention comprises a film case, an anode, a cathode, and an electrolyte solution sealed in the film case, an anode lead wire whose one end is connected to the anode and other end projects outside the film case, and a cathode lead wire whose one end is connected to the cathode and other end projects outside the film case, characterized in that the film case is formed by laminating two packaging films, each packaging film comprising an engineering plastic layer, a water-proof thermoplastic polymer layer, and interposing layers between the engineering plastic layer and the water-proof thermoplastic polymer layer, said interposing layers comprising at least one metallic layer, at least one acid resistant layer, and optionally at least one water-proof thermoplastic polymer layer, and the acid resistant layer contains a thermoplastic polymer and at least one metal compound, selected from the group consisting of magnesium oxide and hydrotalcites, in an amount of about 2 parts by weight to about 25 parts by weight with respect to 100 parts by weight of the thermoplastic polymer.
As described above, by laminating the acid resistant layer containing the metal compound on the side of the metallic layer facing the electrolyte solution, any acid produced by the reaction between moisture entering from the outside and the electrolyte solution is absorbed in the acid resistant layer before moving to the metallic layer. That is, the metal compound particles dispersed in the acid resistant layer adsorb the acid produced in the electrolyte solution and consumes the adsorbed acid by reacting with it. Therefore, even when the acid is produced by an invasion of moisture, it is possible to prevent the metallic layer from corroding by this acid or prevent detachment between the metallic layer and the thermoplastic polymer layer.
Water produced in the acid resistant layer by the reaction between the metal compound particles and the acid is also adsorbed in the metal compound particles. Since, therefore, this water produced in the acid resistant layer is prevented from moving from the acid resistant layer to the electrolyte solution, production of more acid in the electrolyte solution is prevented.
The xe2x80x9cacid resistant layerxe2x80x9d means a layer having a function of internally adsorbing acid moving from an external layer as described above, a function of consuming this acid by converting it into water by a chemical reaction, and a function of adsorbing water produced from the acid.
In this acid resistant layer, the content of the metal compound particles is preferably about 2 parts by weight to about 25 parts by weight with respect to 100 parts by weight of the thermoplastic polymer. In this case, the acid adsorbing and decomposing function of the acid resistant layer can be well achieved with no inconvenience, and the strength of the thermoplastic polymer layer can be well maintained. Additionally, the adhesion of the heat-sealed portion of the thermoplastic polymer layer can be well maintained. If the content of the metal compound particles is less than about 2 parts by weight with respect to 100 parts by weight of the thermoplastic polymer, the function of the acid resistant layer can no longer be well achieved. If the content exceeds about 25 parts by weight, the thermoplastic polymer forming the acid resistant layer becomes fragile, and the adhesion to an adjacent layer is weakened to allow easy peeling of the heat-sealed portion.
Also, by laminating at least one water-proof thermoplastic polymer layer containing no metal compound between the acid resistant layer and the electrolyte solution, water produced in the acid resistant layer is adsorbed in the metal compound particles and effectively prevented from moving from the acid resistant layer to the electrolyte solution by the water-repellent effect of the adjacent water-proof layer. Accordingly, it is possible to reliably prevent the water produced in the acid resistant layer from moving to the electrolyte solution to produce more acid.
The xe2x80x9cengineering plasticxe2x80x9d means plastic having excellent mechanical characteristics and high resistance and durability with which the material is used as mechanical parts, electrical parts, and building materials. Examples are polyacetal, polyamide, polycarbonate, polyoxytetramethyleneoxyterephthaloyl, polybutyleneterephthalate, polyethyleneterephthalate, polyimide, and polyphenylenesulfide.
The metal used in the metallic layer is not particularly limited provided that it has corrosion resistance. Although an aluminum foil or an aluminum alloy foil is generally used, it is also possible to use titanium or chromium.
The xe2x80x9chydrotalcitesxe2x80x9d indicate a group including compounds represented by a basic formula Mg6R2(OH)16CO3.4H2O (R=Al, Cr, or Fe). For example, compounds such as hydrotalcite, stichtite, and pyroaurite are used.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.